1. Field of the Invention
The invention generally relates to integrated circuits and more particularly to the fabrication of strained layers of silicon (Si) or of a silicon/germanium (SiGe) alloy, the said integrated circuits generally being used in the fabrication of transistors.
2. Description of the Related Art
The application of a tensile stress to a silicon film results in a considerable increase in the mobility of the carriers.
The techniques currently employed for fabricating strained Si films are entirely based on the epitaxy of silicon or of a silicon/germanium alloy. The first silicon/germanium layer obtained by epitaxy on a bulk silicon substrate is in compression. However, as soon as this first layer exceeds a certain thickness (depending on the germanium concentration and on the deposition temperature), there is strain relief and the silicon/germanium film resumes its “bulk” lattice parameter. The latter is generally called a “relaxed substrate”. Since this lattice parameter is higher than that of silicon, the epitaxial growth of an Si film on this relaxed silicon/germanium layer is again in tension.
Fabrication of transistors on this type of strained silicon film has clearly demonstrated the increase in the performance of the devices thanks to better carrier mobility. However, the fabrication of relaxed silicon/germanium substrates of high crystallographic quality remains a complex process. This is because relaxing the strain in the silicon/germanium film is manifested by the formation of a high dislocation density, which may make the material unusable.
By growing silicon/germanium films having a germanium gradient, the formation of an emergent dislocation at the surface may be avoided. However, in order to be relaxed, the silicon/germanium layer must have a thickness of greater than 1 μm (depending on the germanium concentration). This results in very lengthy deposition times that are therefore incompatible with industrial use.
In addition, after the deposition phase, the silicon/germanium layer requires a chemical-mechanical polishing step and therefore a cleaning step, which makes the process expensive.
Accordingly, there exists a need for overcoming the disadvantages of the prior art as discussed above.